"Heart cells are among the most densely packed in the body — about 100 million fit into a space the size of a sugar cube. The compact structure crams the cells so close together that they can communicate with one another and beat as one lump. For tissue engineers, however, it poses a tricky hurdle: Pack the cells too tightly and some won’t get proper nutrients; too loosely and they can’t coordinate a beat."

https://stanmed.stanford.edu/innovations-helping-harness-sound-acoustics-healing/